The invention relates to a device useful in the oil-extracting and mining industry, and particularly to a device for the fracture and thermo-gas-chemical treatment of the critical formation zone in subterranean wells of various types, for example: to increase oil and gas output; for dehydration, degassing and methane extraction in coal formations; and for the extraction of metal by underground leaching.
The invention relates specifically to devices using the combustion of solid energy carriers, such as solid rocket fuel, to produce a pulse of hot, high pressure gas. The efficiency of such devices in improving the percolation characteristics of the critical formation zone depends on the number and extent of the fractures created. Those in turn depend on many factors, but primarily on the amplitude, duration, and dynamics of the pressure increase created by the combustion.
Many devices are of this general type have been known and used. They are typically solid-propellant gas generators that are lowered into a well with a cable, and they vary in their design and in their effect on the rock formation. The following publications describe some examples and techniques for their use.
U.S. Pat. No. 3,174,545 discloses a device containing completely encased charges of granular powder, with a burning web up to 1 mm in thickness.
A device with a similar type of charge, working with a packer ring system, is described in J. F. Ruderman and D. A. Northrop, A Propellant-Based Technology for Multiple Fracturing Well bores to Enhance Gas Recovery: Application and Results in Devonian Shale, paper SPE/DOE/GRI 12838 presented at the Unconventional Gas Recovery Symposium, Pittsburgh, Pa., May, 1984. A test of an analogous device in a horizontal test hole is described in R. A. Schmidt, N. R. Warpinski, and P. W. Cooper, In Situ Evaluation of Several Tailored-Pulse Well Shooting Concepts, paper SPE/DOE 8934 presented at the SPE/DOE Symposium on Unconventional Gas Recovery, Pittsburgh, Pa., May 1980.
U.S. Pat. No. 3,422,760 describes a device in which a pulsed effect is achieved by successive ignition of several distinct charges.
U.S. Pat. No. 3,090,436 describes a device in which the gas pressure from the combustion of the propellant causes packers to expand and seal the well above and below a point at which the gas is released, concentrating the pressure in a narrow zone. It is usual for devices of this type to have a metallic or polychlorvinyl case. The pressure pulse produced by these devices is relatively brief (up to several milliseconds), and adjustment of the pressure pulse is effected by mixing powders of different sizes and forms in selected proportions, and by choosing an optimal weight for the charge.
U.S. Pat. No. 4,530,396 discloses a device that consists essentially of two cylindrical charges, end to end. Ignition takes place in a central channel by means of an electric igniter, optionally in combination with a quick-burning linear igniter. The first charge burns rapidly outwards from the center and produces an initial pressure pulse tens of milliseconds in length and of sufficient magnitude to break the rock formation. The second charge burns more slowly, either because it burns only from the end nearest the first charge or because it is of a slower burning material, producing an extended period for which the pressure is lower but sufficient to increase the size of fractures made by the initial pressure pulse. The first charge is made of granular powder encased in a sealed sheath, and the interspace between the powder granules is oil-filled. The lower second charge is made of nitrocellulose. As in the devices mentioned above, the disadvantages of this device include the difficulty of producing completely sealed charges of granular powder, and the possibility that a pressure pulse of small length and with a rapid pressure increase at the leading edge will have an adverse effect on crack formation.
Devices using solid-propellant charges and quick-burning powerful linear igniters are described in U.S. Pat. Nos. 4,683,943 and 5,005,641. In the first device, the charges have a protective external coating. Two propellant charges are spaced apart along the length of the well, and one of the propellant charges includes perforator charges to penetrate the well casing. The two charges are ignited simultaneously. The device thus produces a sudden pulse of pressure from the first charge when the charges ignite, followed by a second pulse when the pressure from the second charge travels along the well to the perforations. In the second device, the charge is in a metallic perforated case. Both devices are distinguished from the ones mentioned earlier in that their pressure pulse can be adjusted only by choosing the weight of the charge. They produce a lower rate of increase of pressure, but a pulse of effective pressure of greater duration, up to 100 ms. (xe2x80x9cEffective pressurexe2x80x9d is a pressure value which produces at least about 80% of the ground pressure necessary to produce artificial fractures.)
Operation of all of the gas generators mentioned above is characterized by a high rate of rise of the fracture-forming stress, in excess of 104 MPa/s. In the opinion of American researchers, that causes multiple fractures to be formed. See Pioneering New Concepts in Wifelike Conveyed Stimulation and Surveillance, Hi Tech Natural Resources, Inc, 1991; and R. P. Swift and A. S. Kusubov, Multiple Fracturing of Boreholes By Using Tailored-pulse Loading, SPE Journal, 1982, No. 12, pp. 923-932.
A solid-propellant gas generator design is known, in which a detonation ignition system extends along the entire length of a central channel charge, which is used to increase the rate of rise of pressure and to form multiple fractures. See Haney B., Cuthill D., Technical Review of the High Energy Gas Stimulation Technique, Computalog Ltd, 1996. In this design, a powerful ignition pulse from fuse detonation products creates a developed system of new burning surfaces in charges. In consequence, the rate of rise of stress reaches 105 to 106 MPa/s, which produces typically 4 to 10 fractures in the rock seam. The charge diameter, amount of propellant, and energy of the ignition system energy vary, and can be optimized to get the best results in a particular well. The pulse length of effective pressure is generally from several ms to 100 or 200 ms. The charge is usually encased in a steel perforated case to be run into a well. However, since possibilities for adjusting the duration of effective pressure are limited, the length of fractures formed does not exceed 5 to 7 meters. Increasing the amount of propellant charge in the gas generator assembly leads to sharp increase in the peak explosion pressure and as a consequence to possible structural damage to the well.
Well pressure accumulators (WPA) are known in which a gas generator, consisting of an assemblage of bare charges of significant weight and length, is fired from below and above simultaneously by electric coils built-in on the ends of primary charges. See Tchazov G. A. and Azamatov V. V., Thermo-chemical Effect on Stripper and Complicated Wells, M., Nedre, 1986.
The absence of a pulse from a thermal igniter and the long time taken for the combustion front to spread upwards from the bottom of the well lead to a low rate of gas formation and a pressure pulse of long duration with a rate of increase of pressure, typically             ⅆ      P              ⅆ      t        ≤            10      2        ⁢          xe2x80x83        ⁢    to    ⁢          xe2x80x83        ⁢          10      3        ⁢          xe2x80x83        ⁢    MPa    ⁢          /        ⁢          s      .      
to 102 to 103 MPa/s. Gas generators of this type are mainly used for their effect on the zone of the rock formation adjoining the well, to clean colmatage out.
The use of bare charges with great initial burning surface and small burning web thickness gives substantially greater ability to adjust the rate of increase of pressure. For example, the use of charges with slots in the propellant mass or multi-tubular packaged pieces can increased       ⅆ    P        ⅆ    t  
to 104 MPa/s. See USSR Inventor""s Certificate No. 1704513 A1. However, the pulse length of effective pressure and, in consequence, the length of the fractures formed, remain inadequate with these constructions.
A gas generator with a detonation combustion system of tubular charges consisting of mixed solid propellant is proposed in Russian Federation Patent No. 2,018,508. Every charge has a combustion-inhibiting liner on the outside and a thin-walled metallic tube in the central channel. A detonating fuse extends the length of the generator assembly and is connected with a hermetically sealed explosive cartridge. This quick-burning gas generator generates comparatively high pressures in a short time, and breaks multiple fractures in the rock formation.
A similar gas generator in which the pressure pulse length can be adjusted is proposed in Russian Federation Patent No. 2,047,744. This generator has similar ignition charges in one or several groups. Over or between the ignition charges are propellant charges which have a thick-walled metallic tube in the central channel and which are ignited by hot gases from the ignition charges. The pulse length of effective pressure can be adjusted from ones to several hundreds of ms. A disadvantage of this gas generator is obstruction of the well by residues of the primary charge metallic tubes, which are broken by the detonation fuse into strips with tom edges and can render the well impassable for equipment to be used in further exploration. This generator is also highly metal-intensive and needs expensive mixed fuels.
Powder pressure generator PGDBK-100M is described in Russian Federation Patent No. 933,959. More than ten-thousand wells have been treated with this device in the Russian Federation and CIS countries. It is a precursor of the present invention. The PGDBK-100M generator consists of tubular powder charges with an inhibiting liner on the external surfaces. In the generator assembly, one of the central charges is a primary charge, in a central channel of which there is a completely enclosed metallic tube with an electric fuse and grains of a pyrotechnic compound. Powder grains are also installed in the central channel for the carrier cable in the remaining charges to increase burning surface.
The amount of the tubular powder charge depends on well conditions, collector type, and its mechanical and collecting properties, and is calculated by computer simulation based on nomograms and curves. See Beliaev B. M., Gribanov N. I. et al., Operating Instructions for Pressure Powder Generators in Wells, M., VIEMS, 1989, the entire contents of which are herein incorporated by reference.
A major disadvantage of the pressure generator described above is the low rate of increase of pressure with time, and the lack of opportunity to adjust that rate. The amount of the propellant charge is also limited by the structural strength of the well. Another disadvantage of that design is the metal-cased ignition device. Imperceptible moisture ingress into the device can lead to failure of the moisture-absorbing pyrotechnic grains to ignite.
Furthermore, ignition devices of that sort provide a xe2x80x9csoftxe2x80x9d ignition mode of the primary charge at the expense of local heating of the metallic tube to high temperatures. The xe2x80x9csoftxe2x80x9d ignition mode produces a pulse with a rate of increase of pressure of the order of                     ⅆ        P                    ⅆ        t              ≤                  10        2            ⁢              xe2x80x83            ⁢      to      ⁢              xe2x80x83            ⁢              10        3            ⁢              xe2x80x83            ⁢      MPa      ⁢              /            ⁢      s        ,
which is not very high and tends to form an isolated pair of fractures in the rock seam.
To sum up this review of known solid-propellant gas generator designs which are capable of generating high pressure sufficient to form cracks, the following may be noted.
U.S. gas generator developments using granular powders and solid propellant have led to the creation of encased devices generating rapid, short-lived pulse pressures, forming a multitude of small length fractures in rock.
Russian gas generator developments have led to the creation of bare-charge devices using solid-propellant tubular charges, after the combustion of which essentially only the load-carrying cable is retrieved to the surface.
Gas generators with quickly and slowly during charges are known which are respectively able to form a multitude of short fractures or isolated long fractures.
An object of the invention is to provide a solid-propellant gas generator with a pressure pulse that can be adjusted for well optimization, and which is capable of increasing the rate of rise of pressure in a well.
In a gas generator according to one aspect of the invention, between a primary charge located at the bottom of the gas generator assembly and a known length (Hinh) of clad charges, there is a defined length (H) of bare tubular powder charges or other charges with a large initial burning surface.